LES of the atmospheric boundary layer over water surfaces partially covered with sea-ice
Responsible: Guido Harbusch
Project type: ​DFG project within the joint Antarctica Research Programme ("koordiniertes Programm Antarktisforschung")
Duration: 01/07/1996 - 30/09/2000

This project used PALM simulations to investigate how the turbulent structure of the atmospheric boundary layer is influenced by small scale surface inhomogeneities with typical diameters of about the boundary layer height z_i. The inhomogeneities were designed as discontinuous, chessboard-like variations of the surface heat flux that was prescribed as the bottom boundary condition. A large heat flux amplitude was chosen to create conditions typical of the Arctic or Antarctic marginal ice zone during cold air outbreaks. One main aim of this study was to determine the effects of such large heat flux variations and to describe the influence exerted by variations of incident flow speed and direction. Particular interest was paid to the inhomogeneity-induced secondary circulations that were analyzed using phase averages.

Compared with continuous inhomogeneities of the same size, discontinous inhomogeneities principally cause similar, but much stronger effects. In contrast to a homogeneously heated boundary layer the horizontally averaged second and third moments are considerably affected by the inhomogeneities. Significant effects, however, only occur when the diameter of the inhomogeneities is at least as large as the height of the boundary layer. Then, especially the vertical energy transport increases, and the effects of the inhomogeneities start to show up even in the vertical temperature and heat flux profiles that don't exhibit the shapes typical of a homogeneous CBL any longer. The vertical gradient of potential temperature is slightly positive within the entire mixing layer, and the heat flux profile departs from its usual linear shape. These changes, though, are not due to the discontinuous form of the inhomogeneities but mainly due to the large heat flux amplitude.

The structure of the secondary circulations, however, is very sensitive to the wavelength and form of the inhomogeneities as well as the heat flux amplitude, wind speed and direction. The main controlling parameter is the near-surface temperature distribution and hence the horizontal pressure gradient perpendicular to the mean flow direction. The secondary circulations vary from direct circulations with updraughts over the centers of the heated areas to rather indirect circulations with updraughts on both sides of the centers and downdraughts just above them. Roll-like circulation pattern occur for a background flow of more than 2.5 m/s.

Previous studies often showed that even a moderate background flow of 5 m/s completely eliminates all potential effects of the surface inhomogeneities. In contrast, the present study demonstrated that the influence due to the increasing background flow strongly depend on the orientation of backgraound flow relative to the surface inhomogeneity. On the one hand, even under an incident flow of 7.5 m/s secondary circulations remain strong provided that the wind direction is parallel to the diagonal direction of the chess-board pattern. On the other hand, the effects of the inhomogeneities are considerably weakened even for background velocities as slow as 2.5 m/s provided that the wind vector of the backgroundflow is perpendicular to the side lines of the chess-board squares. (See also the studies by Letzel and Herold.)